Results of cultures from patients with nonspecific urethritis (NSU) during a 4-year period were retrospectively analyzed. NSU cultures included chlamydiae and mycoplasmas. Seventy-five of 2,408 cultures run by the Missoula Community Hospital microbiology laboratory were positive for chlamydiae (3%). Seventy-five of 1,944 mycoplasma cultures were positive for Mycoplasma hominis (4%), 532 of 1,944 for Ureaplasma urealyticum (27%), and 305 of 1,944, for both (15%), for a total of 46% culture positive for any genital mycoplasma. Results of a prospective study of 40 NSU patients' cultures were tabulated separately. From these 40 patients, no chlamydiae were isolated, but the cultures were 15% positive for M. hominis, 42% positive for U. urealyticum, and 10% positive for both. These studies suggest that the prevalence of chlamydiae may vary with patient population and geographic area. In our population, the mycoplasmas appear to be over 15 times as common (3% vs. 46%).